Method and circuit for decoding an analog audio signal using the BTSC standard

ABSTRACT

During the manufacture of a television receiver or a video recorder in which data signals encoded by the BTSC standard are decoded, a precise adjustment is necessary because of parts tolerances. As a result

[0001] The invention relates to a method and a circuit for decoding an SIF signal using the BTSC standard.

[0002] In the United States, stereo sound in television and video is transmitted and processed using the BTSC standard established by the Broadcast Television System Committee. This standard is also known as multichannel television sound, MTS for short.

[0003] According to the BTSC standard, the sum R+L and the difference L−R of the signal R of the right stereo channel and the signal L of the left stereo channel are formed at the transmitter. The sum signal and the difference signal are further processed and encoded into separate branches so that an encoded sum signal and an encoded difference signal are formed that are decoded again at the receiver in order to obtain a stereo audio signal.

[0004] The sum signal R+L, the amplitude-modulated difference signal L−R, a pilot signal, the frequency-modulated Second Audio Program, SAP for short, and the frequency-modulated professional channel are combined into the so-called MTS signal

[0005] At the transmitter, a BTSC encoder encodes a multiplex signal that has been compressed using the DBX noise suppression described in U.S. Pat. No. 4,539,526.

[0006] According to the BTSC standard, the difference signal L−R is amplitude-modulated with twice the video line frequency. Because of the parabolic noise distribution in the transmission of frequency-modulated signals, the noise level in the transmission channel on which the difference signal L−R is transmitted is approximately 15 dB higher than in the transmission channel on which the sum signal L+R is transmitted, although the difference signal L−R is modulated by 6 dB more than the sum signal. For this reason, a dynamic noise suppression system is provided for the channel to transmit the difference signal L−R, e.g. the abovementioned DBX noise suppression system. However, because a DBX noise suppression system is provided in only one channel, the channel for transmission of the difference signal, it must meet strict requirements regarding accuracy and scaling. Even small inaccuracies result in a deteriorated separation of the stereo sound. In order to avoid these disadvantages, a noise suppression system of this kind would be required in both channels, because the errors in dematricization are compensated and therefore act only on the frequency response. Because of the high expense, however, a second noise suppression system is not used.

[0007] The basic principle of the DBX noise suppression system is the so-called masking. If a signal level in one spectral range is much greater than the noise level in this range, a listener will not notice the noise in this range. In order to make the noise inaudible, the DBX noise suppression system encodes the signal so that the signal level of the transmitted signal is much greater than the noise level of the transmission channel. In the DBX noise suppression system, the audio signal to be transmitted in various spectral ranges is compressed so that the level of the transmitted signal is sufficiently high relative to the noise level and the signal energy is distributed as uniformly as possible over the entire frequency range of the signal.

[0008] In order to be able to meet both requirements, a DBX encoder described in the abovementioned U.S. Pat. No. 4,539,526 is equipped with first and second compression stages.

[0009] In the first compression stage, a variable pre-emphasis filter amplifies the high-frequency signal component as a function of the energy in this spectrum in order thus to achieve a distribution of the signal energy over the entire frequency spectrum that is as uniform as possible. The variable pre-emphasis filter is controlled by a control variable obtained by feeding its output signal back through a high-pass.

[0010] In the second stage, the level of the signal is raised or lowered as a function of the total energy by means of a compressor. The compressor is in the form of a controllable amplifier whose control signal is derived from the output signal of the variable pre-emphasis filter by means of a bandpass.

[0011] The control signal for the variable pre-emphasis filter is derived from the high-frequency components, while the control signal for the controllable amplifier is derived from the entire spectrum.

[0012] According to the BTSC standard, at the transmitter, a carrier with a frequency of 4.5 MHz is frequency-modulated with the MTS signal, which must be adjusted precisely to a specific level that always relates to a certain frequency modulation deviation. The modulated MTS signal is the sound intercarrier frequency signal, abbreviated SIF. Thus for example it is defined that the bandpass of the encoder has an amplification of 0 dB when a signal with a frequency of 300 Hz is applied at the input which has modulated the carrier with a deviation of 4.495 kHz. Known analog frequency modulation demodulators supply an output voltage that is proportional to the frequency of the frequency-modulated input signal. The output voltage of the frequency modulation demodulator however is also proportionally dependent on an unknown factor which varies as a result of internal component tolerances and is subject to drift as a result of temperature changes and component aging. Since no known relationship exists for this reason between the frequency modulation deviation and the output voltage of the frequency modulation demodulator, the input level for each module must be very accurately adjusted by means of a potentiometer for example. As a result of the abovementioned drift caused by component aging, an adjustment must be made after a few years in order to achieve good stereo reproduction once again.

[0013] Hence the goal of the invention is to design a method and a circuit for decoding an analog audio signal according to the BTSC standard in such fashion that all the adjustments, both adjustment during manufacture and also subsequent adjustment caused by drift due to component aging, become superfluous.

[0014] The invention achieves this goal according to the method by the fact that initially a digital SIF signal is provided in a first step, is demodulated in a second step to produce the digital MTS signal, and that the digital MTS signal is decoded in a third step using a digital BTSC decoder.

[0015] The invention achieves this goal devicewise by the fact that a digital SIF signal is applied at the input of a digital frequency modulation demodulator whose output is connected with the input of a digital BTSC decoder, at whose outputs the sum signal R+L and the difference signal L−R can be obtained.

[0016] The invention will now be explained with reference to the figures.

[0017]FIG. 1 shows an embodiment of the invention and

[0018]FIG. 2 shows a circuit according to the prior art.

[0019] In FIG. 2, a tuner T receives a signal A from an antenna. The output signal of tuner T is the so-called Sound Intercarrier Frequency Signal, abbreviated as the SIF signal and is a known term in the BTSC standard. The SIF signal is a carrier at 4.5 MHz frequency-modulated with the MTS signal. The MTS signal consists of the following components: the sum signal R+L, a pilot signal, the amplitude-modulated difference signal L−R, the frequency-modulated Second Audio Program, abbreviated SAP, and a frequency-modulated professional channel. The output signal SIF of the tuner T is demodulated in an analog frequency modulation demodulator AFD whose output signal, the MTS signal, is supplied through an equalizing device, for example a potentiometer P, to an analog BTSC decoder, at whose outputs the sum signal R+L and the difference signal L−R can be obtained. Each device is adjusted using potentiometer P.

[0020] The invention will now be explained with reference to the embodiment shown in FIG. 1.

[0021]FIG. 1, a tuner T receives the output signal A of an antenna. The output of the tuner is connected with the input of a digital frequency modulation demodulator DFD whose output is connected with a digital BTSC decoder DD, at whose outputs the sum signal R+L and the difference signal L−R can-be obtained. The digital frequency modulation demodulator DFD is composed of an analog/digital converter AD and a processor PO connected in series therewith.

[0022] Tuner T receives an analog input signal A from an antenna. The output signal of tuner T, the SIF signal, is initially converted in the digital frequency modulation demodulator DFD by the analog/digital converter AD into a digital SIF signal, from which the processor PO, using the reversing function of the frequency modulation modulator at the transmitter, calculates the MTS signal DS in digital form, whose components are the sum signal R+L and the amplitude-modulated difference signal L−R. Following digital demodulation, there are clearly defined amplitude ratios in the digital demodulated MTS signal DS. In the digital BTSC decoder DD, the digital MTS signal DS is broken up into its components, the sum signal R+L and, by amplitude demodulation, the difference signal L−R. The function of a digital frequency modulation demodulator is known.

[0023] The function SIF of the modulator is as follows:

SIF=a0*cos(w0*t+dOmega*Integral (0, t) (v(k)*dk))

[0024] with w0=4.5 MHz/2/Pi,

[0025] with dOmega=deviation

[0026] with v(k)=function to be transmitted, in this case the MTS signal

[0027] and with a0=amplitude of SIF

[0028] The MTS signal is calculated by calculating the reversing function of the above function from the digitized SIF signal.

[0029] The difference signal L−R is calculated by the function of the DBX expander, while the sum signal R+L is calculated by means of a fixed deemphasis.

[0030] Part tolerances or drift caused by aging or temperature variations are no longer involved in the method according to the invention, since a specific frequency modulation deviation always leads to a precisely defined digital amplitude. The digital MTS signal DS at the output of the digital frequency modulation demodulator DFD is supplied directly to the digital BTSC decoder DD. Because both the digital frequency modulation demodulator and the digital BTSC decoder are digital components, exact level ratios exist between them, independently of the external wiring. Compensation is therefore no longer necessary. The digital frequency modulation demodulator and the digital BTSC decoder can be made advantageously as an integrated circuit.

[0031] The invention is suitable for television receivers and video recorders that process video signals according to the BTSC standard. In contrast to the prior art, all adjustments are eliminated, resulting in faster and less expensive production. In addition, no readjustments due to drift as a result of component aging are any longer necessary. The invention combines in a surprising fashion the advantage of faster and more economical manufacturing with a higher product quality.

[0032] Within the scope of the present invention and in conjunction with FIG. 1, it was described that an analog SIF signal SIF is provided at the output of tuner T and this SIF signal SIF is then digitized in the digital frequency modulation demodulator DFD. However, the invention also includes the concept that the video signal is present in digital form or is digitized and the digital SIF signal SIF is obtained from this digital signal mixture. List of references A output signal of an antenna T tuner AFD analog frequency modulation demodulator P potentiometer D analog BTSC decoder DFD digital frequency modulation demodulator AD analog/digital converter PO processor DS digital MTS signal DD digital BTSC decoder MTS analog MTS signal SIF SIF signal 

1. Method for decoding an SIF signal (SIF) encoded by the BTSC standard, characterized in that a digital SIF signal (SIF) is provided in a first step, this digital SIF signal (SIF) is demodulated in a second step to produce a digital MTS signal (DS), and the digital MTS signal (DS) is decoded in a third step by means of a digital BTSC decoder (DD).
 2. Method for decoding an SIF signal (SIF) encoded according to the BTSC standard according to claim 1, characterized in that an analog SIF signal (SIF) is present and is then digitized.
 3. Circuit for decoding an SIF signal (SIF) encoded using the BTSC standard, characterized in that an analog/digital converter unit (AD) is provided to produce a digital SIF signal (SIF), in that the digital SIF signal (SIF) is supplied to the input of a digital frequency modulation demodulator (DFD), whose output is connected with the input of the digital BTSC decoder (DD), at whose output the sum signal R+L and the difference signal L−R can be obtained.
 4. Circuit according to claim 3, characterized in that the digital frequency modulation demodulator (DFD) includes an analog/digital converter (AD) and a processor (PO) connected in series therewith.
 5. Circuit according to claim 3, characterized in that the digital frequency modulation demodulator (DFD) and the digital BTSC decoder (DD) can be in the form of an integrated circuit.
 6. Circuit according to claim 4, characterized in that the digital frequency modulation demodulator (DFD) and the digital BTSC decoder (DD) can be in the form of an integrated circuit. 